I've come across it full to the point of barely being able to board a train at Tyndrum or Crianlarich heading South in summer - forget the trolley, even if it is onboard somewhere it is not coming to you and you most definitely aren't getting to it on such trains. A big plus of the 153's (unless I've missed something) is an extra toilet for not many more seats, frequently on summer services absurd queues for the toilets occur, 1 per two over full 156 coaches just doesn't cut it and they frequently don't last the full trip on busy services!
The basic concept behind the 153 refurb is sound, their introduction is welcome, the specifics of operation and ticketing may need changed, but the big change that is needed is the one to the timetable to allow these 153s to also head to Fort William.
Anyone suggesting coaches as an alternative to the WHL, try driving the A82 Loch Lomondside in the height of summer or bad weather in winter!